debug: Pass --gdwarf-N to assembler if fixed gas is detected during configure

> > As for the test assembly, I'd say we should take
> > #define F void foo (void) {}
> > F
> > compile it with
> > gcc -S -O2 -g1 -dA -gno-as-loc-support -fno-merge-debug-strings
> > remove .cfi_* directives, remove the ret instruction, change @function
> > and @progbits to %function and %progbits, change .uleb128 to just .byte,
> > I think all the values should be small enough, maybe change .value to
> > .2byte and .long to .4byte (whatever is most portable across different
> > arches and gas versions), simplify (shorten) strings and adjust
> > sizes, and do something with the .quad directives, that is dependent on
> > the address size, perhaps just take those attributes out and adjust
> > .debug_abbrev?  Finally, remove all comments (emit them in the first case
> > just to better understand the debug info).
>
> I'm afraid it is hard to avoid the .quad or .8byte.
> Here is a 64-bit address version that assembles fine by both x86_64 and
> aarch64 as.
> Unfortunately doesn't fail with broken gas versions with -gdwarf-2 without
> the nop, so we'll need at least a nop in there.
> Fortunately gcc/configure.ac already determines the right nop insn for the
> target, in $insn.
> So I guess what we want next is have the 32-bit version of this with .4byte
> instead of .8byte and just let's try to assemble both versions, first
> without -gdwarf-2 and the one that succeeds assemble again with -gdwarf-2
> and check for the duplicate .debug_line sections error.

Ok, here it is in patch form.
I've briefly tested it, with the older binutils I have around (no --gdwarf-N
support), with latest gas (--gdwarf-N that can be passed to as even when
compiling C/C++ etc. code and emitting .debug_line) and latest gas with Mark's fix
reverted (--gdwarf-N support, but can only pass it to as when assembling
user .s/.S files, not when compiling C/C++ etc.).

2020-10-07  Jakub Jelinek  <jakub@redhat.com>

	* configure.ac (HAVE_AS_GDWARF_5_DEBUG_FLAG,
	HAVE_AS_WORKING_DWARF_4_FLAG): New tests.
	* gcc.c (ASM_DEBUG_DWARF_OPTION): Define.
	(ASM_DEBUG_SPEC): Use ASM_DEBUG_DWARF_OPTION instead of
	"--gdwarf2".  Use %{cond:opt1;:opt2} style.
	(ASM_DEBUG_OPTION_DWARF_OPT): Define.
	(ASM_DEBUG_OPTION_SPEC): Define.
	(asm_debug_option): New variable.
	(asm_options): Add "%(asm_debug_option)".
	(static_specs): Add asm_debug_option entry.
	(static_spec_functions): Add dwarf-version-gt.
	(debug_level_greater_than_spec_func): New function.
	* config/darwin.h (ASM_DEBUG_OPTION_SPEC): Define.
	* config/darwin9.h (ASM_DEBUG_OPTION_SPEC): Redefine.
	* config.in: Regenerated.
	* configure: Regenerated.

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